1. Field of the Invention
The present invention relates to a thermistor device that can detect temperatures in the range from room temperature to a high temperature of approximately 1000.degree. C., namely a wide-range thermistor device, along with a method of manufacturing such a thermistor device, and is particularly suited to a temperature sensor for automobile exhaust gases.
2. Description of the Related Art
Thermistor devices for temperature sensors are used in the measurement of temperatures in the range 400-1300.degree. C., namely from the middle to high temperature range, in the measurement of automotive exhaust gas temperatures, gas flame temperatures in gas water heaters and the like, and the temperatures of heating ovens.
The resistance value and the resistance-temperature coefficient (temperature dependence of the resistance value) indicate the characteristics of this type of thermistor device. Here, in order for the temperature detection circuits that make up a temperature sensor to correspond to a practical resistance value range, the resistance value of the thermistor device is preferably within a stipulated range. For this reason, perovskite-type materials and the like are mainly used as the materials that have the resistance value characteristics suited to a wide-range thermistor device.
Examples of proposed thermistor devices that use perovskite-type materials include those disclosed in the unexamined published Japanese patent applications JP-A-6-325907 and JP-A-7-201528. In order to achieve a thermistor device that is usable in a wide temperature range, oxides of Y, Sr, Cr, Fe, Ti and other elements are mixed in a stipulated composition ratio, ground, granulated and fired to obtain a thermistor device that is a perfect solid solution by means of the solid-phase method.
The resistance value and the resistance-temperature coefficient represent the resistance-value characteristics of a thermistor device. In consideration of the resistance-value range of the temperature detection circuit in a normal temperature sensor, the resistance value of the thermistor device must be 50.OMEGA. to 100 k.OMEGA. in the service temperature range. In addition, if a thermistor device is subjected to a heat history of room temperature to 1000.degree. C. or the like, it is preferable that the change from the initial resistance value to the resistance value after the that history be as small as possible.
Each of the aforementioned unexamined Japanese patent publications (Kokai) proposes thermistor devices consisting of various perfect solid solutions, but only the data on the thermistor resistance value at 300.degree. C. or higher is disclosed. For this reason, the present inventors investigated the resistance value characteristics of the various thermistor devices in the various unexamined Japanese patent publications (Kokai) at temperatures near room temperature.
As a result, those thermistor devices that exhibited stable resistance value characteristics in a heat history from room temperature to 1000.degree. C. or the like showed an elevated resistance value in the region from room temperature to 300.degree. C., being undistinguishable from a resistor so the temperature could not be detected. On the other hand, those thermistor devices that satisfied the low resistance value requirement of 50.OMEGA. to 100 k.OMEGA. exhibited a change in the resistance value in the heat history that varied by 10% or more with respect to the initial resistance value so they lacked stability.
In any case, there was no thermistor device (a so-called wide-range thermistor device) that satisfied both of the two conflicting resistance characteristics of a low resistance value characteristics in the range from room temperature to 1000.degree. C. and low resistance value stability in a heat history.
In light of the aforementioned problems, the first object of the present invention is to provide a thermistor device that has stable characteristics and exhibits a small change in its resistance value even in a heat history from room temperature to 1000.degree. C. or the like, and also has a resistance value of 50.OMEGA. to 100 k.OMEGA. in the temperature range from room temperature to 1000.degree. C.
On the other hand, in recent temperature sensors for automobile exhaust gas (exhaust temperature sensors), the demand for incorporating these exhaust temperature sensors into systems for detecting the exhaust gas temperature before and after the catalyst in order to detect deterioration of the catalyst used for change the exhaust gases from gasoline-powered vehicles non-toxic, and into systems for detecting the exhaust temperature before and after the catalyst in order to control the catalyst temperature for cleaning up the exhaust gases, particularly NOx, from diesel-powered vehicles, is great.
However, temperature sensors that use conventional thermistor devices have a large amount of dispersion in their resistance-temperature characteristic (R-T characteristic), for example, their temperature accuracy is +20-30.degree. C. (room temperature to 600.degree. C.), which is not sufficient for high-precision system control. Thus, in order for them to be incorporated into such systems, the development of thermistor devices that can achieve higher temperature accuracy is desirable. To this end, the present inventors have performed diligent studies of the structure. Manufacturing methods and other aspects of conventional thermistor devices, and as a result found the following problems with increasing the temperature accuracy.
To wit, in the aforementioned solid-phase method, the mixing and grinding of a plurality of oxide materials is performed by means of a ball mill or the like, but because of inadequate grinding capacity, the average grain size of the thermistor materials after grinding has a limit of 1-2 .mu.m, so uniform mixing is not achieved. In addition, because of this difference in the average grain size of the oxide materials, the reactions do not proceed uniformly in the calcining, sintering and other heat treatments, so the thermistor raw material composition produced by the thermal reactions becomes non-uniform.
Moreover, in the aforementioned mixing and grinding operations performed by ball mill or the like, the constituent elements of the alumina balls or the like in the grinding media become mixed into the thermistor materials as impurities, thus causing resistance dispersion or divergence from the desired composition of the thermistor device. Therefore, the resistance-value dispersion of the thermistor device obtained becomes large and a temperature sensor that uses this thermistor device will have dispersion in its R-T characteristic, leading to degraded temperature accuracy.
In passing, in the current state, the aforementioned systems cannot be built with the temperature accuracy of temperature sensors that use conventional thermistor devices, so high-accuracy but high-cost thermocouples or platinum resistors or the like are used as the temperature sensors instead. However, in either case, as of now there are no temperature sensors that use thermistor devices that have a temperature accuracy (e.g., .+-.5.degree. C. over room temperature to 600.degree. C.) of a level that can be applied to the aforementioned systems and the like.
To this end, in light of the aforementioned problems, the second object of the present invention is to provide a method of manufacturing a thermistor device from a sintered body such that the dispersion of the resistance value of the thermistor device is reduced.
On the other hand, when the aforementioned thermistor device is used in a temperature sensor that detects the temperature of automobile exhaust gas, the thermistor device located at the detector tip of the temperature sensor is typically covered with a metallic cap for the purpose of preventing the accumulation of debris, soot or the like from the gas being monitored. Here, since the metallic cap is subjected to thermal oxidation due to the heat of the hot exhaust gas at roughly 900.degree. C., the interior of the metallic cap is a reducing environment, and thus the thermistor device inside is subjected to a reducing action and its resistance value may change, causing problems.
For this reason, temperature sensors are typically placed in electric furnaces and subjected to thermal aging treatment at 900-1000.degree. C. for 100 hours. However, while the temperature sensor is in service, in the event that air should enter the interior of the cap due to a hole in the cap or the cap becoming loose, there is a risk that the thermistor device itself will again be subjected to a reducing environment and the aforementioned change in the resistance will occur.
In JP-A-9-69417, even though special metallic materials, e.g. Inconel 600.TM. or other materials, are selected for the metallic cap and machining is performed, the problem of the change in resistance value is still not solved in the case in which the thermistor device itself is subjected to a reducing environment.
In either case, there has not been provided a thermistor device that exhibits resistance-value stability when the thermistor device itself, in a temperature sensor or the like is subjected to a reducing environment.
In light of the aforementioned problems, the third object of the present invention is to provide a device structure that has resistance-value stability even in the case when the thermistor device is subjected to a reducing environment.